Apache generally logs all the web server
access events in the access_log file, whereas
errors and warnings go into the error_log file.
The access_log file can later be analyzed to
report server usage statistics, such as the number of requests made
in different time spans, who issued these requests, and much more.
The error_log file is used to monitor the server
for errors and warnings and to prompt actions based on those reports.
Some systems do additional logging, such as storing the referrers of
incoming requests to find out how users have learned about the site.

The simplest logging technique is to dump the logs into a file opened
for appending. With Apache, this is as simple as specifying the
logging format and the file to which to log. For example, to log all
accesses, use the default directive supplied in
httpd.conf:

This setting will log all server accesses to a file named
/home/httpd/httpd_perl/logs/access_log using the
format specified by the LogFormat
directive—in this case, common. Please refer
to the Apache documentation for a complete explanation of the various
tokens that you can use when specifying log formats. If
you're tempted to change the format of the log file,
bear in mind that some log analysis tools may expect that only the
default or one of a small subset of logging formats is used.

The only risk with log files is their size. It is important to keep
log files trimmed. If they are needed for later analysis, they should
be rotated and the rotation files should be moved somewhere else so
they do not consume disk space. You can usually compress them for
storage offline.

The most important thing is to monitor log files for possible sudden
explosive growth rates. For example, if a developer makes a mistake
in his code running on the mod_perl server and the child processes
executing the code start to log thousands of error messages a second,
all disk space can quickly be consumed, and the server will cease to
function.